1. Field of the Invention
The present invention relates to ultrasonic sensors, and more particularly, to an ultrasonic sensor used, for example, for a back-up sensor for an automobile.
2. Description of the Related Art
An ultrasonic sensor of the related art used for back-up sensors of automobiles is attached to a bumper or other suitable structure of the automobiles, and is used as an obstacle detection sensor, such as a back-up sensor or a corner sensor. The ultrasonic sensor is attached to the bumper such that a bottom portion of a case member having a piezoelectric element fixed thereto is substantially perpendicular to a road surface and the ultrasonic sensor is located and adjusted in a direction in which ultrasonic waves are emitted. In an ultrasonic sensor used for such an application, if the range of ultrasonic wave transmission and reception in a horizontal installation direction is too narrow, a dead angle occurs in the detection range. If the range of ultrasonic wave transmission and reception in a vertical direction is too broad, a reflection of waves from the ground surface causes noise. Therefore, anisotropy in directional properties in the horizontal and vertical installation directions is required.
FIGS. 9A to 9C include schematic diagrams showing an example of a case member 1 used for an ultrasonic sensor as described above. FIG. 9A is a cross-sectional plan view of the case member 1, FIG. 9B is a cross-sectional view taken along a line B-B (in a vertical installation direction) shown in FIG. 9A, and FIG. 9C is a cross-sectional view taken along a line C-C (in a horizontal installation direction) shown in FIG. 9A. The case member 1 is composed entirely of a metal material, such as aluminum, and is provided with a hollow portion 3 which is open toward the rear. A bottom portion 2 of the case member 1 includes a thick portion 2a at the center thereof in the vertical installation direction, and substantially crescent-shaped thin portions 2b at both sides thereof. One electrode surface of a piezoelectric element 5 is bonded to an inner surface of the thick portion 2a at the center of the bottom portion 2 by an electrically conductive adhesive or other suitable adhesive. In the cross section in the vertical installation direction, therefore, as shown in FIG. 9B, the thin portions 2b are located at either side of the thick portion 2a having the piezoelectric element 5 mounted thereon. In the cross section in the horizontal installation direction passing through the center of the case member 1, as shown in FIG. 9C, the entire bottom portion 2 is defined by the thick portion 2a. The thick portion 2a has a thickness greater than a minimum thickness of an outer peripheral sidewall portion 4 of the case member 1, and the thin portions 2b have a thickness less than the minimum thickness of the outer peripheral sidewall portion 4 of the case member 1.
The ultrasonic sensor having the structure described above narrows the transmission and reception range in the vertical installation direction (the direction in which the width of the hollow portion 3 extends). Since there is a difference between the transmission and reception range in the horizontal installation direction and the transmission and reception range in the vertical installation direction, an ultrasonic sensor having anisotropy in directional properties is obtained (see, for example, Japanese Unexamined Patent Application Publication No. 2000-32594).
However, the sidewall of the case member 1 in the ultrasonic sensor described in Japanese Unexamined Patent Application Publication No. 2000-32594 is provided with a thick portion and thin portions to achieve desired directional properties, and the case member 1 with such a complex structure is manufactured by processing aluminum, such as forging, cutting, and die casting (high-pressure casting). Due to the complexity of the structure, the manufacturing cost is high.
Another problem is as follows. It is preferable that the surface of the case member 1 to which the piezoelectric element is adhered has a structure that ensures a sufficient degree of vibration. In particular, it is preferable that a portion (corner/edge) defined between the bottom surface and sidewall of the case member 1 vibrates. However, in the ultrasonic sensor of Japanese Unexamined Patent Application Publication No. 2000-32594, the case member 1 is provided with a thick portion and thin portions, and the vibration in the vicinity of the thick portion is suppressed. It is therefore difficult to achieve significant anisotropy.
Still another problem is as follows. The case member 1 in the ultrasonic sensor of Japanese Unexamined Patent Application Publication No. 2000-32594 is designed such that the hollow portion of the case member 1 has an elliptical cross section in order to ensure anisotropy in directional properties. The case member 1 formed into an elliptical shape has a thin sidewall portion, and the amplitude of side vibration is relatively large at that portion. As a result, for example, if the ultrasonic sensor is mounted in an automobile, characteristics of the ultrasonic sensor are likely to change when a rubber cushion and a housing are secured to the automobile. Therefore, it is difficult to ensure desired characteristics.